Mammalian ovarian follicles produce and secrete steroid hormones which regulate the function of reproductive system components. Fates of ovarian follicles are: 1) degeneration (atresia) and 2) ovulation, luteinization, and corpus luteum formation. Studies have shown in the pig and other species that the terminal steps in follicular development are: 1) loss of ability to produce the follicular steroid product estrogen (E) followed by 2) acquisition of capability to produce the luteal steroid product progesterone (P). This phenomenon can be termed E-P shift. These terminal steps appear to occur both in atresia and in the ovulation/uteinization transition. Preliminary data in this application show that one component of the pig follicile (granulosa cells) exhibit the same sequence of terminal steps when placed in vitro. Since the initial step in the E-P shift is loss of E production, it is possible that the signal responsible for this loss induces both atresia and the ovulation/luteinization transition. In the requested support period, this project will investigate control of E production by pig granulosa cells as follows: 1) Develop a complete description of characteristics of the porcine granulosa cell in vitro model for the E-P shift with regard to loss of E production. The methodology employed will identify metabolites of the substrate for E production and the relative importance of process with may divert this substrate. 2) After obtaining the description of the E-P shift in vitro, evidence will be sought which supports the same phenomenon in vitro in the pig ovary. The methodology employed will be histochemical examination of ovaries for evidence of a shift in follicular enzyme activity from E-P production in folliciles undergoing atresia and preovulatory luteinization. In addition, follicular antral fluid steroids will be examined for evidence of the E-P shift. 3) Studies will be initiated into the mechanism which controls the loss of E production capability. Various hormonal mechanisms will be investigated using the in vitro model to identify the signal which initiates the E-P shift. The future course of the project will involve: 1) further efforts to identify the signal which causes loss of E production capability, and 2) study of the specific intracellular mechanisms which mediate actions of this signal. By developing and studying a in vitro model for the E-P shift, this project will provide insight into the mechanisms of atresia and ovulation/luteinization transition.